Processing of substrates by means of focused charged particle beams is a well-established technique in a wide range of technological areas, in particular, semiconductor manufacturing. Often, it is desired to perform processing of structures on a semiconductor device, where these structures are buried beneath several microns of material (such as silicon) which is opaque to visible light. Near-infrared (NIR) imaging has an advantage of being able to penetrate through these layers, however, with reduced spatial resolution due to the longer wavelengths of NIR light. Visible light has some ability to penetrate these layers, as well.
One example of a charged particle beam process is back-side circuit editing, applicable to flip-chip devices, where the only way to access internal regions of the devices in the circuit is by removing material from the back of the chip, typically with focused ion beam (FIB) milling. After a sufficient amount of material has been milled away, the circuit layers can be imaged using visible light to locate the exact device positions for charged particle beam processing, such as cutting and adding interconnects.
Another example of a charged particle beam process is front-side circuit editing. Often, the layers of interest for processing may be beneath 1-5 μm of silicon, which is largely opaque to visible light (for λ<1.1 μm, corresponding to the bandgap energy of silicon). When bright-field imaging (where the illumination is normal to the substrate surface) is attempted using visible light, there is generally too much absorption to enable imaging of these buried structures, plus, reflected light off the substrate surface interferes with light scattered from within the device, resulting in loss of image contrast. Using dark-field imaging (where the visible light illumination of the device is at a glancing angle to the substrate), imaging is possible, since the reflected light from the substrate surface does not contribute to the overall image.
Thus, there is a need for both near-infrared imaging (with superior depth penetration through silicon) and visible light imaging (with superior spatial resolution due to the shorter wavelength) for use in locating structures within semiconductor devices which are to be processed with a charged particle beam.
In some systems combining both optical imaging for navigation (i.e., locating areas for beam processing) and charged particle beam processing columns, the imaging and processing subsystems are integrated together within a small volume, where both the imaging and processing may be performed without the need for substrate motion. A generally serious limitation of these implementations is that the imaging and processing subsystems physically interfere with each other due to their respective diameters. Also, it is not possible for both imaging and processing to be perpendicular to the substrate surface. Both these disadvantages tend to limit the achievable spatial resolutions, both for imaging and for the subsequent beam processing steps. Thus, alternative system designs have been used in which the axes of the imaging subsystem and the charged particle column are separated and the substrate is moved between the two subsystems, alternatively being imaged and then processed, often over many cycles, where the imaging process serves for both initially locating regions before processing begins and then for endpoint detection during processing. In these implementations with physically separated imaging and processing subsystems, it is obviously necessary to know the separation of these two subsystems very precisely.
Structures near or at the surface of a substrate, such as a microcircuit, may not be easily imaged using the charged particle beam. The difficulty in charged particle beam imaging may arise due to lack of sufficient image contrast, or due to the fact that the charged particle beam may induce damage, such as milling or contamination, as a result of the imaging process. Thus, it is useful to have an imaging process that does not damage the substrate prior to processing. In some charged particle beam processing systems, an optical imaging capability is integrated into the same physical region of the system as the charged particle beam. However, in these systems, there is typically a difficulty in optimizing either the imaging or the beam processing due to physical interference between the imaging and processing subsystems. Often this results in increased working distances for both the imaging and processing subsystems, resulting in loss of spatial resolution for both imaging and processing.